A Quantum Mechanically Derived Force Field To Predict CO2 Adsorption on Calcite 10.4 in an Aqueous Environment

A Silvestri and A Budi and E Ataman and MHM Olsson and MP Andersson and SLS Stipp and JD Gale and P Raiteri, JOURNAL OF PHYSICAL CHEMISTRY C, 121, 24025-24035 (2017).

DOI: 10.1021/acs.jpcc.7b06700

Density functional theory (DFT) with semiempirical dispersion corrections (DFT-D2) has been used to calculate the binding energy of a CO2 molecule on the calcite 10.4 surface for different positions and orientations. This generated potential energy landscape was then used to parametrize a classical force field. From this, we used metadynamics (MTD) to derive free energy profiles at 300 and 350 K for CO2 binding to calcite, CO2 binding with Ca2+, and pairing of two CO2 molecules, all for aqueous conditions. We subsequently performed classical molecular dynamics (MD) simulations of CO2 and water on the 10.4 surface at pressures and temperatures relevant for CO2 geological storage. Density profiles show characteristic structured water layering at the calcite surface and two distinct phases of water and CO2. We have also calculated the densities of the CO2-rich and water-rich phases and thereby determined the mutual solubilities. For all the pressures and temperatures in the studied range, CO2 was unable to penetrate the ordered water layers and adsorb directly on the solid surface. This is further confirmed by the free energy profiles showing that in the presence of water there is neither direct adsorption to the 10.4 surface nor contact binding of CO2 with Ca2+. Rather, we saw a weak affinity for the surface of the ordered water layers. At 5 MPa and 323 K, we observed the nucleation of a CO2 droplet located above two structured water layers over the solid. It could not penetrate the structured water but remained bound to the second water layer for the first 10 ns of the simulation before eventually detaching and diffusing away.

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